With respect to a miniaturized vehicle such as a motorcycle, it has been known that a swing aim is formed using a light alloy such as an aluminum alloy or a magnesium alloy for enhancing the reduction of weight to make the product more attractive. In a miniaturized vehicle disclosed in JP-A-2008-81107, a swing arm is integrally formed by die-casting using an aluminum alloy.
In forming a swing arm by casting, when a wall thickness of the swing arm is increased for increasing the rigidity of the swing arm, a cooling rate of a thick wall portion becomes slow resulting in a drawback wherein cavities occur in the portion. Thus, a simple increase in the wall thickness cannot be adopted. More particularly, with respect to a swing arm on which a shock absorber is supported, it is desirable to make the occurrence of cavities difficult while increasing the rigidity of a support boss that is formed on a cross portion for supporting the shock absorber.